Cassini Significant Events
11/12/08 - 11/18/08
The most recent spacecraft telemetry was acquired on Nov. 18 from the Deep
Space Network tracking complex at Goldstone, California. The Cassini
spacecraft is in an excellent state of health and all subsystems are
operating normally. Information on the present position and speed of
the Cassini spacecraft may be found on the "Present Position" page
at: http://saturn.jpl.nasa.gov/operations/present-position.cfm.
Wednesday, Nov. 12 (DOY 317):
Beginning today and running through Nov. 18, science activities are part of
a Magnetic Field Investigation (MAG) segment. So what's a segment? After
the mission extension was approved following a NASA Senior Review in
February of 2007, the flight team began looking at how to divide up the tour
after Prime Mission from 2008 to 2010. The first step was to define science
objectives for each of the disciplines and instruments. A reference
trajectory was developed and delivered incorporating the science goals and
locking in the targeted flybys, orbits, and science opportunities. After
this, Science Planning (SP) went to work to define the guidelines on how all
the science observations would be integrated together. If all the science
teams had to participate in the integration of the entire tour, the
workload would have been tremendous. Instead, each orbit was divided into
"segments." Each segment is integrated by one of the Target Working Teams
(TWT) or Orbiter Science Teams (OST): MAG, Rings, Saturn, Targeted Flyby,
Titan Orbiter Science Team (TOST), or Satellite Orbiter Science Team (SOST).
For the prime mission, almost the entire four-year tour was integrated
before any sequence development began. For Extended Mission (XM) each
integration delivery for each sequence - containing multiple segments - is
only a few days prior to the start of development for that sequence, and
approximately six months prior to the start of execution.
The MAG orbit 93 segment includes periapsis on DOY 321, a highly contested
time for science activities. Many disciplines desired observations in this
segment: there were two distant icy satellite opportunities, Tethys and
Mimas; a ring occultation; an aurora campaign for the Magnetospheric and
Plasma Science (MAPS) instruments, and observations for the Saturn TWT. In
addition Radio Science (RSS) had a request for an experiment in XM to
determine Saturn's gravity field and infer constraints on its internal
structure. Details of what experiments actually occurred when follow later
in this report.
Orbit Trim Maneuver (OTM) #171 was performed today. This is the apoapsis
maneuver setting up for the Titan 47 encounter on Nov. 19. The main engine
burn began at 3:30 PM PST. Telemetry immediately after the maneuver showed
the burn duration was 30.9 seconds, giving a delta-V of 5.14 m/s, as
planned. All subsystems reported nominal performance after the maneuver.
An encounter strategy meeting was held today to cover the period from Nov.
19 to Dec. 5, which includes Titan flybys T47 and T48 and maneuvers 173-175.
Thursday, Nov. 13 (DOY 318):
The MAG segment began today with a Titan cloud monitoring observation,
followed by a MAPS survey with the Cassini Plasma Spectrometer (CAPS) as the
prime instrument. The Visual and Infrared Mapping Spectrometer (VIMS) was
then active for 10 hours and performed an apoapsis mosaic of the unlit side
of Saturn's rings.
The S45 DOY 329 Live Inertial Vector Propagator (IVP) Update process for
Saturn/Helene/Tethys was kicked off following this morning's Status and
Scope meeting. Preliminary pointing analysis based on the OTM-171 solution
is available for team review. The OTM-172 orbit determination solution will
be used for this update and will be available by 4pm on Saturday, Nov. 15.
Friday, Nov. 14 (DOY 319):
DOY 319 began with a Mimas observation by the Ultraviolet Imaging
Spectrograph (UVIS) and Imaging Science (ISS), followed by a long
compositional observation by the Composite Infrared Spectrometer (CIRS) to
measure oxygen compounds - H2O, CO2 - in the Saturnian stratosphere as a
function of latitude. An ISS movie of the F-ring was fit in during a
transfer from a Canberra station to one in Madrid for data downlink.
Saturday, Nov. 15 (DOY 320):
The short version is, OTM-172 has been cancelled. Now as to the "why"...
Based on tracking data through this morning, OTM 172 had a magnitude of
about 6 mm/sec. A bias of 0.3 sec in the time of T47 closest approach would
be required to achieve an acceptable maneuver magnitude of about 12 mm/s.
The comparison between this maneuver and the no-maneuver case indicated a
saving of about 0.3 m/s by OTM 172 cancellation. Science has evaluated the
pointing errors that would result from not doing the maneuver based on the
preliminary Navigation results, and no adverse effects were found. Since
the no-maneuver trajectory today is closer to the nominal than the analyzed
trajectory, no additional science analysis is required. Therefore OTM 172,
scheduled for execution on Nov. 16, is cancelled. The no-maneuver files and
plots will be provided to the flight team. The DOY 329 Live IVP Update
process will still continue as planned. Science Planning has taken a look
at the effects of the cancellation; they are very small.
Sunday, Nov. 16 (DOY 321):
A non-targeted flyby of Tethys occurred today.
The first observation period today was short with a hi-resolution ISS F-ring
stare on the day of periapsis. This was designed to fit with a
CIRS/VIMS/UVIS eclipse of Dione by Saturn, looking for spatial variations in
the response of the surface temperature to the abrupt change in sunlight.
Next was an RSS Saturn gravity experiment with a downlink pass. The RSS
activity window fell over the Canberra/Madrid view period handover, so
multiple DSN stations were scheduled. In addition, the Cosmic Dust Analyzer
(CDA) requested some high-data-rate time for an hour over periapsis. To
achieve this the downlink was interrupted and a 1-hour observation block in
telemetry mode SnER-2 was added.
The RSS observation was the third Saturn flyby in the mission dedicated to
the determination of Saturn's gravity field, and the first in extended
mission. The RSS team has already determined the coefficients J2, J4 and J6
with good accuracy. The orbit 93 results will be combined with those of
orbit 28 from September 2006 and orbit 68 from May 2008 to improve the
determination of these coefficients. This will allow RSS to infer
constraints on the interior of Saturn. The activity consisted of one 7hr
30min segment, which included periapsis passage. This was the last RSS
observation for this year.
The SP analysis package for the DOY 329 live IVP update has now been posted.
SP will be recommending a "Go" for four of the observations that fall within
the live IVP update time frame. Two of the four observations are CIRS
observations which will require an update to the Saturn vector. For these
observations, the aim is to measure the temperature of the same territory on
both the lit and unlit sides of the rings and determine the energy balance.
Monday, Nov. 17 (DOY 322):
DOY 322 adds more CAPS prime time and a CIRS observation designed to look at
both the lit and unlit sides of the rings at similar geometries to study the
vertical thermal distribution within the rings. How much sunlight is
absorbed as it passes through the rings? Answering this question will help
in understanding ring dynamics and ring particle properties.
Tuesday, Nov. 18 (DOY 323):
DOY 323 finds time for a long CIRS observation to probe the thermal spectrum
of the rings. From prior observations it is known that there is a roll-off
in ring temperature at sub millimeter wavelengths. Characterizing this
roll-off would provide information on the properties of the ring particle
regoliths and how they vary within the rings. Finally, there is an Optical
Remote Sensing (ORS) collaborative observation of the Saturn south pole for
auroral activity. The MAPS instruments also pick up their data rates for the
auroral crossing. To close out the MAG segment, VIMS monitored Titan for
clouds.
All S46 Instrument Expanded Block (IEB) loads going to the SSR or going
direct to the instrument were approved for uplink. The first IEB uplink
window opens today and the second window on DOY 325. All twelve IEB loads
are scheduled for uplink on these two windows. In addition, S46 will be the
first sequence to use a PRIMO SSR repair at the end of every scheduled IEB
uplink window to ensure that all loads are on the SSR without any memory
corruption caused by an SSR swap. Take a look at last week's entry on
Monday, Nov. 10 (DOY 315), for more information on this topic.
The preliminary results are in and 79 semifinalist essays from 110 students
representing 22 states have been selected for Cassini's Scientist for a Day
- Fall 2008 program. All semi-finalists and their teachers and classmates
have been invited to participate in teleconferences with Cassini scientists
on Dec. 9 and 10, 2008.
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